Studies have continued on determining the processes governing tissue levels of superoxide dismutase (SOD) and other potential longevity determinant processes. We have completed the project investigating the role the major histocompatibility complex may play in determining tissue levels of SOD. Cu/Zn and Mn types of SOD were measured in the liver of 10 differnt H-2 congenic strain mice. No significant correlation was found with H-2 type nor with their lifespan. We have also completed studies determining the SOD levels in liver of parabiosed mice of different H-2 congenic mouse strains to observe possible effects of humoral factors on SOD tissue levels. No effect is evident. Finally, SOD levels in liver of rats have been determined that had been exercised voluntarily throughout their lifespan. Results show no enhancement of SOD levels as compared to non-exercised controls.